The present invention relates to tobacco, and in particular, to the post-harvest treatment of tobacco.
Nitrosamines are known to be present in air, foods, beverages, cosmetics, and even pharmaceuticals. Preussman et al., Chemical Carcinogens, 2nd Ed, Vol. 2, Searle (Ed.) ACS Monograph 182, 829-868 (1984). Tobacco and tobacco smoke also are known to contain nitrosamines. Green et al., Rec. Adv. Tob. Sci., 22, 131 (1996). Tobacco is known to contain a class of nitrosamines known as tobacco specific nitrosamines (TSNA). Hecht, Chem. Res. Toxicol., 11(6), 559-603 (1998); Hecht, Mut. Res., 424(1,2), 127-142 (1999). TSNA have been reported to be present in smokeless tobacco, Brunnemann et al., Canc. Lett., 37, 7-16 (1987), Tricker, Canc. Lett., 42, 113-118 (1988), Andersen et al., Canc. Res., 49, 5895-5900 (1989); cigarette smoke, Spiegelhalder et al., Euro. J. Canc. Prev., 5(1), 33-38 (1996); Hoffmann et al., J. Toxicol. Env. Hlth., 50, 307-364 (1997); Borgerding et al., Food Chem. Toxicol., 36, 169-182 (1997); nicotine-containing gum, Osterdahl, Food Chem. Toxic., 28(9), 619-622 (1990); and nicotine-containing transdermal patch, Adlkofer, In: Clarke et al. (Eds.), Effects of Nicotine on Biological Systems II, 17-25 (1995).
Green and freshly harvested tobaccos have reported to be virtually free of TSNA. Parsons, Tob. Sci., 30, 81-82 (1986); Spiegelhalder et al., Euro. J. Canc. Prev., 5(1), 33-38 (1996); Brunnemann et al., J. Toxicol.-Clin. Toxicol., 19(6and7), 661-668 (1982-3); Andersen et al., J. Agric. Food Chem., 37(1), 44-50 (1989); Djordjevic et al., J. Agric. Food Chem., 37, 752-756 (1989). However, it has been observed that TSNA form during the post-harvest processing to which tobacco is subjected. Tricker, Canc. Lett., 42, 113-118 (1988); Chamberlain et al., J. Agric. Food Chem., 36, 48-50 (1988). TSNA are recognized as being formed when tobacco alkaloids, such as nicotine, are nitrosated. Hecht, Chem. Res. Toxicol., 11(6), 559-603 (1998). There has been considerable effort expended toward studying the mechanism of formation of TSNA.
Significant efforts have been expended towards studying the mechanism of TSNA formation during tobacco curing, particularly for Burley tobacco. As a result, it has been postulated that TSNA form during the air-curing of Burley tobacco as a result of microbial mediated conversion of nitrate to nitrite, and the subsequent reaction of nitrate-derived chemical species with alkaloids present in the tobacco. Hamilton et al., Tob. Sci., 26, 133-137 (1982); Burton et al., J. Agric. Food Chem., 40, 1050-1055 (1992); Bush et al., Coresta Bulletin Information, Abstract 9814 (1995); Wiernik et al., Rec. Adv. Tob. Sci., 21, 39-80 (1995); Cui et al., TCRC (1996). It also has been suggested that the mechanism by which TSNA form during the flue-curing of Virginia tobaccos is similar to that mechanism postulated for air-cured Burley tobacco. See, Djordjevic et al., J. Agric. Food Chem., 37, 752-756 (1989) and Peele et al., Coresta Bulletin Information, Abstract 9822 (1995). See also, PCT WO 98/05226 and PCT WO 98/58555, and U.S. Pat. No. 5,803,801 to O""Donnell et al.
It has been known practice to cure certain types of tobaccos, particularly specialty tobaccos, using a so-called fire-curing process. Legg et al., TCRC (1986). It also has been common practice to flue-cure certain tobaccos, such as Virginia tobaccos, in barns using a so-called flue-curing process. Cooper et al., VPI Bull., 37(6), 3-28 (1939); Brown et al., Agric. Eng., 29(3), 109-111 (1948); Johnson et al., Tob. Sci., 4, 49-55 (1960); Peele et al., Rec. Adv. Tob. Sci., 21, 81-123 (1995). Tobacco leaf is harvested, placed in barns, and subjected to the application of heat. In recent years, it has been common practice, particularly in North America, to cure tobacco using a so-called direct-fire curing technique. Typical direct-fire heating units are powered by propane, and during use, those heating units produce exhaust gases that come into contact with the tobacco being cured. As a result, it is common for tobacco being cured to be exposed to propane combustion products, including nitric oxides that are present in those exhaust gases; and it is not uncommon for tobacco within a curing barn to be exposed to about 0.5 to about 2 kilogram of nitric oxide during a typical curing cycle of about 6 days in duration.
Tobaccos of a particular type that are cured using flue-curing techniques have been reported to provide higher levels of TSNA relative to similar tobaccos of like type that are air-cured. Chamberlain et al., Beitr. Taobak., 15(2), 87-92 (1992). Furthermore, potential relationships between so-called direct-fire heating techniques and the formation of nitrosamines have been investigated in industries outside of the tobacco industry. IARC Monograph, 17, 35-47 (1978); Stehlik et al., Ecotoxicol. Envir. Saf., 6, 495-500 (1982); Scanlan et al., In: Loeppky et al. (Eds.) Nitrosamines and Related N-Nitroso Compounds, 34-41 (1994). However, direct-fire heating techniques have not always been associated with the formation of nitrosamines. Larsson et al., Swedish J. Agric. Sci., 20(2), 49-56 (1990). In addition, those references have not reported any correlation between contact of tobacco with nitric oxides during curing and levels of TSNA in direct-fire flue-cured tobacco. However, it has been observed that TSNA form during the flue-curing processes commonly employed in the curing of Virginia tobaccos. Peele et al., Rec. Adv. Tob. Sci., 21, 81-123 (1995).
Typically, in North America, tobaccos such as Virginia tobacco are flue-cured using curing barns equipped with direct-fire heating units that burn propane. However, tobacco flue-curing using curing barns equipped with heat exchange units that burn diesel fuel have been employed to a limited degree within North America. Heat exchangers currently are employed to a significant extent outside of North America, particularly where coal and wood are the prominent fuels. Teague et al., Coresta Bulletin Information, Abstract 9824 (1995). For example, curing barns equipped with heat exchange units have been employed in countries including Japan, Turkey, Brazil and Zimbabwe. Tobacco curing barns equipped with non-direct-fire heating units, such as heat exchange units, provide for a so-called indirect heating of the tobacco being cured; and as such, when indirect heating techniques are used to flue-cure tobacco, exhaust gases generated by the heat source do not come into contact with that tobacco to any significant degree.
Attempts have been made to reduce the TSNA levels within tobacco. For example, it has been suggested that control of the temperature and moisture during air-curing may have an effect upon lowering TSNA levels within air-cured tobaccos, such as Burley tobacco. See, IARC Monograph, 84, 451-455 (1986). It has been proposed to process tobacco to remove TSNA; such as by the manner that is described in U.S. Pat. No. 5,810,020 to Northway et al. It also has been proposed to cure tobacco in conjunction with the application of microwave radiation and high temperature treatment in order to provide a tobacco possessing extremely low TSNA levels. See, PCT WO 98/05226 and PCT WO 98/58555, and U.S. Pat. No. 5,803,801 to O""Donnell et al.
It would be desirable to provide a manner for treating tobacco in order that TSNA levels within that tobacco are provided at very low levels. It would be particularly desirable to provide an efficient and effective manner or method for inhibiting TSNA formation within tobacco during curing, and particularly during flue-curing.
The present invention relates to the curing of tobacco and cured tobaccos. Of particular interest is a tobacco curing method that provides a suitably cured tobacco that possesses extremely low levels of tobacco specific nitrosamines (TSNA). Preferred tobaccos that are processed in accordance with the present invention are Virginia tobaccos, and the preferred method for curing those tobaccos is the flue-curing method.
In one aspect, the present invention involves flue-curing tobacco under conditions such that the tobacco that is being subjected to cure is subjected to minimal contact with gaseous nitric oxides. Thus, in a preferred embodiment, steps are taken to avoid contact of tobacco being flue-cured with exhaust gases produced by the heating units that provide the source of heat for the flue-curing process. The present invention allows for a method to prevent formation of TSNA during curing, and allows for tobaccos so cured to possess significantly reduced levels of TSNA relative to similar tobaccos similarly cured using direct-fire curing techniques.
In another aspect, the present invention relates to a method for curing tobacco using a tobacco curing barn. The method is carried out for the purpose of providing cured tobacco possessing an extremely low TSNA content. The method involves the steps of: providing green tobacco; placing the green tobacco in the curing barn; subjecting the tobacco to application of heat from a heating source that is not a direct-fire heating source under conditions suitable to cure that tobacco; and avoiding contact of the tobacco during curing with nitric oxide gases, such as those nitric oxide gases that are present in the exhaust gases of direct-fire heating units (e.g., those by products of combustion of propane). Thus, the method involves activities than are purposefully taken to avoid contact with, or exposure to, tobacco being subjected to flue-curing processing steps with nitric oxide gases.
In yet another aspect, the present invention relates to a method for modifying a tobacco curing barn for the purpose of providing conditions suitable for curing tobacco such that resulting cured tobacco obtained from such barn possesses a TSNA content lower than that of similar tobacco cured using that barn prior to such modification. The method involves the steps of: providing a tobacco curing barn equipped with a direct fire heating unit; removing the direct fire heating unit from the barn; equipping the barn with a heating unit that does not provide contact of tobacco within the barn with nitric oxide gases (e.g., as are provided by exhaust gases containing nitric oxide by products of combustion, such as are provided by the combustion of propane) during cure; and equipping the barn such that the barn is operational for tobacco cure.